Callbacks and non-stdio I/O

Although stdio is convenient and nearly universally implemented as per ANSI C, it is not suited to all or even most potential uses of Vorbis. For additional flexibility, embedded applications may provide their own I/O functions for use with Vorbisfile when stdio is unavailable or not suitable. One common example is decoding a Vorbis stream from a memory buffer.

Use custom I/O functions by populating an ov_callbacks structure and calling ov_open_callbacks() or ov_test_callbacks() rather than the typical ov_open() or ov_test(). Past the open call, use of libvorbisfile is identical to using it with stdio.

Read function

The read-like function provided in the read_func field is used to fetch the requested amount of data. It expects the fetch operation to function similar to file-access, that is, a multiple read operations will retrieve contiguous sequential pieces of data, advancing a position cursor after each read.

The following behaviors are also expected:

a return of '0' indicates end-of-data (if the by-thread errno is unset)
short reads mean nothing special (short reads are not treated as error conditions)
a return of zero with the by-thread errno set to nonzero indicates a read error

Seek function

The seek-like function provided in the seek_func field is used to request non-sequential data access by libvorbisfile, moving the access cursor to the requested position. The seek function is optional; if callbacks are only to handle non-seeking (streaming) data or the application wishes to force streaming behavior, seek_func and tell_func should be set to NULL. If the seek function is non-NULL, libvorbisfile mandates the following behavior:

The seek function must always return -1 (failure) if the given data abstraction is not seekable. It may choose to always return -1 if the application desires libvorbisfile to treat the Vorbis data strictly as a stream (which makes for a less expensive open operation).
If the seek function initially indicates seekability, it must always succeed upon being given a valid seek request.
The seek function must implement all of SEEK_SET, SEEK_CUR and SEEK_END. The implementation of SEEK_END should set the access cursor one past the last byte of accessible data, as would stdio fseek()

Close function

The close function should deallocate any access state used by the passed in instance of the data access abstraction and invalidate the instance handle. The close function is assumed to succeed; its return code is not checked.

The close_func may be set to NULL to indicate that libvorbis should not attempt to close the file/data handle in ov_clear but allow the application to handle file/data access cleanup itself. For example, by passing the normal stdio calls as callback functions, but passing a close_func that is NULL or does nothing (as in the case of OV_CALLBACKS_NOCLOSE), an application may call ov_clear() and then later fclose() the file originally passed to libvorbisfile.

Tell function

The tell function is intended to mimic the behavior of ftell() and must return the byte position of the next data byte that would be read. If the data access cursor is at the end of the 'file' (pointing to one past the last byte of data, as it would be after calling fseek(file,SEEK_END,0)), the tell function must return the data position (and thus the total file size), not an error.

The tell function need not be provided if the data IO abstraction is not seekable, or the application wishes to force streaming behavior. In this case, the tell_func and seek_func fields should be set to NULL.

copyright © 2007 Xiph.org	Ogg Vorbis
Vorbisfile documentation	vorbisfile version 1.2.0 - 20070723